(1) Field of the Invention
The present invention relates to a method of automatically regulating an aircraft power plant, to a device, and to an aircraft.
More particularly, the invention is applicable to a rotary-wing aircraft.
(2) Description of Related Art
Conventionally, a rotary wing aircraft is equipped with a power plant comprising at least one engine such as a piston engine or a turbine engine. Such a turbine engine may also be referred to as a “turboshaft engine”.
A gearbox connects the power plant to the main advance and lift rotor: this is referred to as the “main gearbox” or “MGB”.
Temperature limits for an engine and torque limits for an MGB serve to define an operating envelope for each engine that covers two normal utilization ratings:                a takeoff rating corresponding to a level of torque for the MGB and a level of heating for the engine that can be accepted for a limited length of time without significant degradation, this takeoff rating being defined by a maximum takeoff power PMD and by a duration for using this maximum takeoff power that is generally of the order of five minutes; and        maximum continuous rating, which rating is defined by a maximum continuous power PMC corresponding to about 90% of the maximum takeoff power PMD, and by a utilization duration for said maximum continuous power that is generally unlimited.        
In addition, manufacturers define an idling rating for minimizing fuel consumption, with the engine nevertheless continuing to keep running while idling.
The idling rating for an aircraft engine is a particular mode of operation enabling the engines of the aircraft to operate on the ground while minimizing the nuisance and/or while maximizing the comfort of the people and crew moving around the aircraft. In particular, the idling rating serves to:                keep the engine up to temperature for rapid departure;        minimize the noise emitted by the aircraft;        minimize pollutant emission and fuel consumption; and to        enable electricity to be generated on board and hot air to be taken for the purpose of heating and demisting the cabin.        
The idling rating is therefore a relatively complex mode, having objectives that can be opposing and constrained. For example, the lift rotor of a helicopter must be driven by a turbine engine operating at an idling rating that is relatively low in order to minimize noise, but it is also necessary for the engine to have an idling rating that is relatively high in order to enable an electricity generator to operate.
The ratings enabling the aircraft to operate in flight are, for convenience, referred to as “flight ratings”, whereas the rating enabling the engine to idle is referred to as the “idling rating”.
The aircraft is then provided with a physical state selector having three stable positions. These three positions for the state selector are: engines stopped or “STOP”; engines in idling mode or “IDLE”; and engines in flight mode or “FLY”.
This manual state selector (STOP/IDLE/FLY) thus makes it possible to indicate to an on-board engine computer in the aircraft:                to stop each engine when the selector is in the “STOP” position;        to implement the idling rating when the selector is in the “IDLE” position; and        to implement a flight rating when the selector is in the “FLY” position.        
Therefore, when the pilot positions the selector in the “IDLE” position, the engine computer of an engine regulates said engine so as to cause it to operate in compliance with the idling rating defined by the manufacturer.
In a first example, an engine computer regulates the first speed of rotation Ng of the gas generator of the engine.
Thus, an engine computer acts, in particular, on a fuel metering device of the engine to make the first speed of rotation Ng tend towards a setpoint speed of rotation Ng*.
That first example offers the advantage of guaranteeing a setpoint speed of rotation of the gas generator that enables some minimum amount of mechanical power to be extracted (taken off) and some minimum amount of hot air to be extracted (taken off).
Such a minimum extraction of hot air may be determined to ensure heating and/or demisting of a cabin of the aircraft.
In addition, this first example prevents any untimely takeoff of the aircraft while the idling mode is engaged. If a pilot increases the collective pitch of the blades of the rotary wing, then the power delivered by the engine does not increase. On the contrary, the second speed of rotation of the free turbine and the speed of rotation of the rotor decrease.
Since the second speed of rotation NTL of the free turbine and the first speed of rotation of the rotary wing vary, the noise generated by the aircraft is not controlled. In addition, the rotary wing might find itself within an operating range that might induce a phenomenon of ground resonance.
By way of a variant, in a second example, an engine computer regulates the second speed of rotation NTL of a free turbine of the engine.
Thus, an engine computer acts, in particular, on a fuel metering device of the engine to make the second speed of rotation NTL tend towards a setpoint speed of rotation NTL*.
That second example offers the advantage of ensuring a speed of rotation for the rotor of the helicopter that is constant. The above-mentioned drawbacks are then avoided.
Unfortunately, the first speed of rotation Ng can vary without said first speed being controlled by the regulation system. The first speed of rotation Ng can then become insufficient to enable a minimum amount of mechanical power to be extracted and a minimum amount of hot air to be extracted.
Finally, the setpoint used for the second speed of rotation is generally less than the nominal speed for the rotary wing in flight.
If a pilot accidentally changes the collective pitch of the blades of the rotary wing, the first speed of rotation Ng increases. The power developed by the engine is increased accordingly. The aircraft might then take off with a second speed of rotation that is potentially too low.
Therefore, that state of the art requires pilots to determine in intentional manner whether they wish to implement an idling rating via an idling mode or a flight rating via a flight mode. Depending on the aircraft, the idling rating is, in addition, implemented by regulating the speed of rotation of the gas generators of the engines or by regulating the speed of rotation of the free turbines of the engines.
In addition, if a slight increase in power is necessary for a secondary need (more heating, an increased electricity need, etc.), the pilot must switch the regulation of the engine over to the flight mode of operation.
In the aviation sector, various documents mention automated monitoring and control of operation of a power plant while idling.
Thus, Document US 2011/0208400 describes the use of a selector having an “IDLE” position and a “MAXPOWER” flight position in the context of electronic control for adjusting operation of an aircraft turboprop engine. A man-machine interface thus enables the pilot to choose a mode of operation for the turboprop, between a free power delivery mode of operation and an idling mode of operation.
Weight-on-Wheels (WoW) information from a sensor for sensing that the aircraft is on the ground is taken into account in order to define an idling rating.
Document U.S. Pat. No. 4,500,966 describes “super contingency” control for a helicopter on which the speed of rotation of the main rotor is too low as a result of an engine failure.
Document WO 2000/039442 also describes a system for regulating an airplane or helicopter engine.
The technical background also includes the following documents: U.S. Pat. No. 5,403,155, U.S. Pat. No. 6,880,784, and US 2004/088085.